- *** Contraindications to Adenosine (known hypersensitivity to Adenosine, sick sinus syndrome, second or third degree atrio-ventricular block - except in patients with functioning artificial pacemaker, long QT syndrome has been defined as QTc > 450 ms at baseline). ECG will be undertaken just after the first dose of the study drug and QT/QTc will be recorded and compared to the baseline. If the QTc recorded after the first dose of the study drug exceeds 450ms or there is an increase in the QT/QTc of > 60 ms from baseline, the second dose will be abandoned and this will be recorded.